Rudolf K. Thauer

NADH, a physiological electron donor in clostridial nitrogen fixation

Clostridial nitrogenase catalyzes the ATP dependent reduction of Ns to NHs with reduced ferredoxin as electron donor [l] . In cell-free extracts of N,-grown Clostridium pasteuriunum several systems have been used to regenerate the reduced ferredoxin required in the nitrogenase reaction: pyruvate and pyruvate-ferredoxin oxidoreductase [2] , hydrogen and hydrogenase [3] as well as formate and COZreductase [4]. Therefore pyruvate, hydrogen and formate are generally regarded as the physiological reductants in clostridial nitrogen fixation [5,6]. NADH has also been reported to reduce ferredoxin via a NADH-ferredoxin oxidoreductase in several clostridia, which were however NHs-grown and thus devoid of the nitrogenase system [7-lo]. Since the NADH-ferredoxin oxidoreductase has not been demonstrated in Ns-grown clostridia as yet, NADH does not appear to be accepted as an electron donor for clostridial Ns-reduo tion, although this was indicated by preliminary evidence [3] and theoretical considerations [l I]. In this communication it is shown that NADHferredoxin oxidoreductase is present also in cellfree extracts of Ns-grown Cl. pasteurianum and

The active species of “CO2” utilized in ferredoxin-linked carboxylation reactions

The active species of “CO2” , i.e. CO2 or HCO3−−(H2CO3) utilized by enzymes catalyzing ferredoxin-linked carboxylation reactions was determined. The enzyme investigated was pyruvate synthase from Clostridium pasteurianum (EC 1.2.7.1; Pyruvate: ferredoxin oxidoreductase). Data were obtained which were compatible with those expected if CO2 is the active species.The dissociation constant (KS) of the enzyme-CO2 complex was measured. At pH 7.2 KSfor CO2 of pyruvate synthase was found to be approximately 5 mM.